The present invention concerns a core barrel, in particular for oil exploration, the core barrel having a string, an external tube fixed to the front end of the string, from the point of view of forward travel of the core barrel in a formation, an annular core bit mounted on a front portion of the external tube, and an internal tube assembly, which is mounted in xe2x80x9cwire linexe2x80x9d mode, that is to say mounted in the external tube so as to be able to be retrieved and brought to the surface again through the string, and which has hydrodynamic means arranged to transform a core sampling fluid pressure flowing in the string into a force thrusting said assembly towards the front of the core barrel.
There is a pressing need to improve this type of core barrel, particularly for its use in wells with portions inclined with respect to the vertical or more particularly with horizontal portions, all the more so when these portions are at a great distance from the vertical entry portion of the well. It is in fact known that, in this case, the string is subjected at least locally to compression and therefore to a risk of buckling amplified by an inclined or horizontal positioning. This situation causes the string to rub unnecessarily against the wall of the well, and therefore to wear, to be checked in its forward movement and, where applicable, in its rotation driving the bit.
The purpose of the present invention is to propose a solution to the problem set out above and thus to procure a core barrel on which it is possible for the bit not to be constantly pushed directly by the string, particularly in the horizontal position, for its forward movement in a formation but can be pushed particularly by the pressure of the coring fluids sent to the bottom of the well through the string, the bit however being able to be held up, if necessary, in its advance by this string.
To this end, the core barrel mentioned at the start has according to the invention the following particularities: the front portion of the external tube is mounted for axial sliding, limited by front and rear stops, in a rear portion of the same external tube and projects therefrom longitudinally; the internal tube assembly has a support stop arranged to selectively push forward the front portion of the external tube, and means are arranged between the internal tube proper and the said front portion of the external tube so that the latter can be driven in rotation about its longitudinal axis independently of said internal tube, the latter being able to be held fixed in rotation with respect to the formation during core sampling.
Through this arrangement of the core barrel of the invention, the bit is pushed by the internal tube assembly in the formation to be sampled as long as the front portion is not in abutment against one or other of the stops. Because of this, at least part of the string is not subjected to buckling and is substantially less pressed against the wall of the well being sampled, for example in a bend therein, which would not be the case in a normal core barrel configuration.
According to a preferred embodiment of the core barrel of the invention, the latter has, for rotating the front portion of the external tube and the bit, a motor mounted in a rear part of the internal tube assembly. Means are then arranged between the front and rear portions of the external tube so that one can be driven in rotation about its longitudinal axis independently of the other. The motor stator can be fixed in rotation with respect to the rear portion of the external tube, during core sampling, whilst the motor rotor is then fixed in rotation to the front portion of the external tube.
Through these measures, the string and the rear portion of the external tube are appreciably less subject to wear and to fatigue due to stresses of the type caused by alternating bending of the tubes turning in curves of the well. In addition, the evenness of the rotation of the bit thus driven by the motor is greater than that which would be provided by the string since there is no interference from a rubbing of the strings against the wall of the well.
Advantageously, the motor stator can be kept fixed in rotation with respect to the rear portion of the external tube by at least one assembly consisting of a catch and a corresponding longitudinal groove. Likewise, the rotor can be kept fixed in rotation with respect to the front portion of the external tube also by at least one assembly consisting of a catch and corresponding longitudinal groove. One or other or both of the catches can then be arranged so as to come into engagement in a corresponding groove or to be released therefrom automatically when the internal tube assembly is put in the core sampling position in the external tube or is respectively withdrawn therefrom. This design allows easier fitting and removal of the internal tube assembly in the external tube.
Other technically equivalent means can be used in place of the catch and groove assemblies, such as for example reciprocal flutes.
The motor is preferably of a type actuated by the core sampling fluid, for example a PDM (Positive Displacement Motor) or a turbine known in the art. In this case, the motor can participate in the aforementioned hydrodynamic means, at least for the part of said fluid which actuates it and which is therefore subjected to a pressure drop.
One advantage of a PDM or particularly of a turbine is to be seen in its high rotation speed, and therefore in an advantageous rate of advance, in particular in the case of hard rocks for which preferably bits of the impregnated type or concretion type are preferably used. However, given the small diameter imposed on this type of motor in order to be able to pass through the external tube of the core barrel of the invention, it may be appropriate to choose an extended type or one with several stages in order to obtain sufficient power at the bit. This does not however constitute a handicap since there is a means of easily organising space longitudinally for this purpose.
In one embodiment of the invention, the core barrel can have, to help the internal tube assembly to descend in the external tube, a joint system mounted on the internal tube assembly so as to substantially completely close the annular space between this assembly and the external tube and thus to receive, like a piston, the full pressure and full flow of drilling fluid. Then the joint system can have at least two flat circular joints, whose external edge is divided into a kind of petal, the two joints being arranged one on the other so that a petal on one joint covers a gap between two petals on the other joint.
The core barrel of the invention is thus advantageously arranged to function according to two modes. There is a decoupled mode in which the front portion of the external tube slides freely during core sampling with respect to the string and the rear portion, under the thrust of the internal tube assembly, with the advantages explained above and the additional advantage that variations in the progress of the string in the well are no longer transmitted to the bits. There is also a coupled mode in which the internal tube assembly pulls by means of stops on the rear external tube portion and therefore on the string. At least part of the string then being under traction, and this is advantageous from the point of view of wear and the directional behaviour of complete assembly in the well.